It is known that granules of vermiculite, a common naturally occurring micaceous mineral, may be swollen or expanded many-fold by the action of aqueous salts. Thereafter, the "exfoliated" structures are delaminated by shearing to yield thin platelets termed "vermiculite lamellae." These thin platelets have unique surface charge characteristics. The platelet surface is mostly anionic with localized positive charge at the edge of the particle. The platelets also have high aspect ratios (e.g., diameter:thickness). These properties facilitate the formation of vermiculite papers/films, laminates, coatings, foams, and articles, such as gaskets, which have heat resistance and flame-retardant properties.
U.S. Pat. No. 3,434,917 of Kraus et al., incorporated herein by reference, disclosed a process for preparing sheets. A suspension of vermiculite platelets is first prepared by immersing vermiculite ore in sodium chloride and lithium chloride solutions, rinsing it with water and swelling it, and then shearing it through vigorous agitation of the suspension. Particle size may be further reduced by use of a colloid mill. After adjustment of platelet concentration in the water medium, a flocculation agent, such as potassium hydroxide, hydrochloric acid, various inorganic salts, mineral acids or bases, or the like, is added to aggregate the platelets into a pulp-like mass. The pulp is deposited onto a screen to form a paper mat. Other materials such as clays, cellulose fibers, and polymeric fibers can be added to the suspension prior to flocculation for improving strength and tear resistance. Kraus also teaches a method for forming wet-laid vermiculite paper from a flocculated vermiculite dispersion.
European Patent Application 0,441,996 A1 of Hercules, Inc., published Aug. 21, 1991, disclosed a composite sheet made from a flocculated mixture of mechanically delaminated vermiculite, either glass or paper fibers, and at least one flocculating agent to flocculate the dispersion. The flocculated mixture may be used to form paper handsheets on conventional papermaking screens (e.g., 100 mesh screens) or continuous paper rolls using a 12-inch (30.5 cm) wide flat wire Fourdrinier paper machine.
In U.S. Pat. Nos. 4,269,628 and 4,324,838, incorporated herein by reference, Ballard et al. disclosed that vermiculite sheets could be made by casting an aqueous paste or slurry containing vermiculite and water stability additives such as magnesium oxide or calcium oxide. These patents stated that laminates of either vermiculite sheet or foam can be formed continuously by extruding an aqueous paste or slurry of the vermiculite composition onto a porous mesh carrier belt, and drying the vermiculite with hot air. Thereafter, an isocyanate-based foam mix can be deposited onto the vermiculite; or the vermiculite/carrier could be fed through a laminator as a bottom facing or center core in the formation of a laminate product.
Vermiculite sheet material is typically formed as a laminate, a thick paper mat, or frequently with cellulose or polymeric fibers for structural support. The present invention, on the other hand, is directed to a method for making a "film" rather than a "paper." The term "film" has often been used synonomously to refer to a thin "paper" even though, as a technical matter, paper is made by screening out a sedimented fiber or pulp such as flocced or aggregated vermiculite. In the present invention, however, the term "film" is used to refer to a vermiculite sheet formed by evaporating water from a nonflocculated colloidal dispersion of delaminated vermiculite platelets. The film is therefore a coherent sheet of vermiculite lamellae that has no substantial discontinuities such as would occur in aggregated or fiber-intertwined paper, which has discontinuities between and among fibers or aggregated particles.
It is known that a vermiculite dispersion can be deposited onto a polished granite table or glass surface and removed as a film. The procedure is slow and labor-intensive because water must be evaporated to dry the wet film, and care must be taken during removal to avoid tearing of the dried film. The inventor has discovered that drying by heat or hot air usually causes a "skin" to form on the film surface which traps moisture that is attempting to evaporate from the vermiculite dispersion. Consequently, bubbles or "blisters" tend to arise and rupture, destroying the coherence of the film. This phenomenon, hereinafter referred to as "blistering," is believed to have precluded the possibility of continuously forming a coherent vermiculite film.
Wire mesh, called the fourdrinier, is used in the paper-making arts for continuously forming paper. The paper is formed when fibers interlock and sufficient water is removed through the pores in the screen to allow the paper sheet to hold together. However, such a carrier is inappropriate for forming a vermiculite film. Vermiculite could fall through the mesh if not flocculated or aggregated first into a fibrous or pulped mass; or it would otherwise become embedded into the mesh and inseparable therefrom even if the vermiculite were sufficiently viscous such that it could sit on top of the mesh.
Plastic films such as polyester and mylar might be contemplated as carrier sheets, but these are difficult to work with because vermiculite dispersions do not tend to form films, especially thin ones, on plastic surfaces. The dispersion tends to "bead up" on the plastic surface. If the surface is roughened, such as by using sandpaper, the vermiculite film is not releasable.
Another problem with attempting to find a suitable carrier for vermiculite film is viscosity. If too fluid, the vermiculite dispersion tends to "bead" on the carrier and does not form a film. If too thick, the dispersion cannot be readily worked into a uniform thin film. In addition, vermiculite can only be loaded into an aqueous dispersion up to a maximum total dry weight vermiculite solids content of about 20 percent. Beyond this point, the vermiculite becomes a gel which will not spread easily.
In view of the foregoing disadvantages of the prior art, a novel method, vermiculite film/carrier, and/or vermiculite composition are needed.